Currently, commercial aviation is relatively safe, if we compare the number of accidents to the number of passengers kilometers traveled.
However, the number of accidents is still too high. The media impact of a severe accident is substantial, and significantly slows the development of an industry that is expected to grow in future years.
The measurement of the speed of an aircraft relative to an air mass is essential for safety. Indeed at low altitude and low speed the aircraft may stall, and at high altitude a small range allows the aircraft to remain in the air. Indeed, given the low air density at altitude the aircraft will stall if it does not move at a relatively high speed, while an over speed leads to the destruction of the aircraft.
Furthermore automatic aircraft systems (e.g., autopilot, auto throttle) are connected directly to the indications of speed relative to the ambient air. A false indication of speed can cause serious accidents especially in case of false simultaneous information in several speed sensors.
Several accidents are due without any doubt to a false indication of the speeds probes. In other serious cases, speed misrepresentations are suspected to have been a direct or indirect cause of serious incidents.
The events of a malfunction of current probes are usually held in exceptionally difficult weather conditions, e.g., icing conditions and/or heavy rain and/or high pressure.
Airworthiness Number 2009-0195 issued by the European Aviation and Space Agency (EASA) said that some speed probes must be replaced by others considered more reliable in icing conditions or heavy rain.
“Pitot” probes from the name of their inventor exist for hundreds of years. They were used on ships and improved by Mr. Ludwick Prandtl.
Pitot probes or Prandtl tubes are used routinely on current aircraft both civilian and military. They have an opening at the front of the probe. This opening is connected to a pressure sensor. This pressure is called the total pressure. This total pressure is compared with a static pressure measured on the sensor itself or at another location of the plane. This so-called total pressure is always greater than or equal to the static pressure. Laws of correspondence enable the computation of the airspeed of the plane from the measured values of total pressure and static pressure.
These probes are simple, accurate, but are sensitive to exceptional weather conditions such as heavy icing and/or heavy rain conditions. They are usually heated, but if, for example, the diameter of ice in the air through is greater than the input port of the probe, it is likely that this probe will be clogged and will no longer work properly.
Moreover in general, these probes comprise a discharge water drain. This allows the drainage of a given water flow. If the ingested flow of water by the Pitot tube is higher than the maximum water flow rate of the drain, it is likely that the probe can no longer function. We can cite some intrinsic deficiencies of this type sensor:                Density error: These errors affect the measurement of speed and altitude. This error is due to pressure changes in the atmosphere that are not related to altitude (Meteorology);        Compressibility error: Measurement errors occur when the approximation of the incompressible regime can no longer be made and that the formula calculating the speed no longer applies. This intrinsic error occurs especially at high altitudes, where the speed of sound is less than its value at sea level. These errors become significant for above 10000 feet and above 200 knots speed altitudes. Under these conditions, the speedometer reports less than the actual speed of the aircraft speed;        Mechanical hysteresis due to the properties of the aneroid capsule contained in the measuring instrument. This hysteresis effect can be caused by an abnormal change in the inclination of the apparatus. This error is characterized by a false value temporarily in inclination measurement and during the reversal hysteresis in the altitude measurement and the measurement of the vertical velocity;        Position Error. This error occurs when the static pressure measured by the tube is different from the real atmospheric pressure away from the unit, particularly when the airflow around the device is not equal to the actual speed of the unit. This may be caused by one or more factors: the angle of attack, the weight of the device, the acceleration, and, in the case of helicopters, the airflow created by the movement of the blades. The reading error can be positive or negative depending on the factors involved. The position errors may be a fixed value (which depends only on the model of the device and can be calibrated) and/or a variable value which can arise from mechanical deformations locally changing the airflow, or special flight situations.        
Conventionally, “Pitot” sensors type are used to measure airspeed in aircraft, yet these probes seem to cause problems in some exceptional conditions of highly icing and/or in heavy rains. However, the maximum security required in aviation is not reached in the case where the cause of fault is very exceptional atmospheric phenomena. In fact increasing the number of identical probes placed under the same conditions does not create a true redundancy. It is therefore important in order to further improve the reliability of complete system to remember that it is better to have a true redundancy of subsystems.